1. Field of the Invention
The present invention relates to an interferometer such as a laser interferometer, an exposure apparatus that employs this interferometer, an exposure method that employs this exposure apparatus and an interference length measurement method.
2. Related Background of the Invention
Batch exposure-type (stepper method) or scanning exposure-type (step and scan method and so forth) exposure apparatuses used for the fabrication of semiconductor devices require a high level of exposure accuracy. For this reason, in an exposure apparatuses, a moving mirror is fixed to the side of a reticle stage on which a reticle as a mask is placed and position of which is controlled or to the side of a wafer stage on which a wafer as a substrate is placed and which moves in two dimensions. A measurement beam is irradiated onto the moving mirror by a laser interferometer or other interferometer, whereby the movement amounts of the stages are successively measured, and it is possible to align the stages highly accurately on the basis of these measured values.
FIG. 9 is a constitutional view of a conventional laser interferometer which is used in order to align the stages of the exposure apparatus. As shown in the figure, a laser beam L which is emitted by a laser light source 100, enters a polarizing beam splitter 102. Two polarized components whose polarizing directions lie orthogonal to each another are mixed in the laser beam L. The component which is polarized perpendicular to the incident plane, that is, the s-polarized light component, is reflected by the polarizing beam splitter 102 and travels toward a fixed mirror 106 via a quarter wavelength plate 104 as a reference beam LR. Interference methods include the heterodyne interference method which allows a small frequency difference between the two polarized components, and the homodyne interference method which does not allow a frequency difference. After being reflected by the fixed mirror 106, the reference beam LR passes through the polarizing beam splitter 102 via the quarter wavelength plate 104 as a p-polarized beam, and then enters a photo-detector 108 that comprises a photo-detector element and a photoelectric conversion element.
Meanwhile, the p-polarized light component of the laser beam L that is polarized parallel to the incident plane passes through the polarizing beam splitter 102 and travels toward a corner mirror 112 (moving mirror) via a quarter wavelength plate 110 as a measurement beam LM. The corner mirror 112 is fixed to a sample object, which is constituted so as to be movable in a direction parallel to the optical path of the measurement beam LM emitted from the polarizing beam splitter 102.
The measurement beam LM is reflected by a first reflection plane 112a and a second reflection plane 112b of the corner mirror 112, and thus travels toward the fixed mirror 106. The measurement beam LM, which is reflected by the fixed mirror 106 and thus caused to travel in the reverse direction, travels toward the corner mirror 112, is reflected by the second reflection plane 112b and the first reflection plane 112a of the corner mirror 112, is reflected by the polarizing beam splitter 102 via the quarter wavelength plate 110 as s-polarized light, and then travels on substantially the same path as the reference beam LR before entering the photo-detector 108. By means of photoelectric conversion of the interference beam obtained from the reference beam LR and the measurement beam LM, the photo-detector 108 generates a counting pulse in accordance with the movement amount of the corner mirror 112 and the movement amount of the corner mirror 112 is measured by adding up the counting pulses.
In this conventional interferometer, even in cases where transverse shift occurs due to oscillation or transverse position change of the corner mirror 112 is required, the optical path of the measurement beam LM that enters the corner mirror 112 from the polarizing beam splitter 102 and the optical path of the measurement beam LM that enters the polarizing beam splitter 102 from the corner mirror 112 can be equalized. In other words, in cases where the corner mirror 112 shifts in position (as indicated by the dot-dash line in FIG. 9), the measurement beam LM′ (shown as a broken line in FIG. 9), which is reflected by the first reflection plane 112a and the second reflection plane 112b of the corner mirror 112 before traveling toward the fixed mirror 106 and being reflected by the fixed mirror 106 and the corner mirror 112, enters the polarizing beam splitter 102 via the same optical path as that traveled by the measurement beam in cases where the corner mirror 112 is not shifted in position. An interferometer of this kind is disclosed in Japanese Patent Application Laid open No. 63-44101.
However, as shown in FIG. 10, in a state where the reflection plane of the fixed mirror 106 which reflects the measurement beam LM is tilted relative to the polarizing beam splitter 102, in case where there is a positional shift of the corner mirror 112 (as indicated by the dot-dash line in FIG. 10), a measurement error is generated. That is, as shown in FIG. 11, in case where there is a positional shift of the corner mirror 112 (the optical path is indicated by the broken line, whereas the optical path in case where there is no positional shift of the corner mirror 112 is indicated by the solid line), the position at which the measurement beam LM enters the fixed mirror 106 in case where there is no positional shift of the corner mirror 112 differs from the position at which the measurement beam LM′ enters the fixed mirror 106 in case where there is positional shift of the corner mirror 112, thus causing the measurement error.